Well it's still a 32 bit program so I guess that helps.
Would probably require some porting to make it 64 bit native, but as long as you use the WPARAM, INT_PTR typed and what not correctly it 'should just work'.
Yeah that's the bulk of the work for migrating small Win32 apps. Things escalate when someone has built their own dynamic GUI framework over Win32, used a range of GUI controls, and then built event-driven apps on top of that, it's a lot lol